For example, Japanese Laid-Open Patent Publication No. 2012-091304 and Japanese Patent No. 5474739 disclose techniques relating to off-line teaching of a welding gun performing a welding operation. In the technique described in Japanese Patent Laid-Open No. 2012-091304, teaching data for existing workpieces is diverted as teaching data for similar new workpieces. In the technique described in Japanese Patent No. 5474739, interference between two robots is detected.